1. Field of the Invention
The invention relates to a hydraulic power steering system that includes an oil pump, a flow rate control valve that controls hydraulic fluid that is supplied from the oil pump to a steering assist device, an electric motor that drives the flow rate control valve, and a control unit that controls the electric motor.
2. Discussion of Background
A power steering system described in Japanese Patent Application Publication No. 2006-306239 (JP 2006-306239 A) includes a cylindrical valve body, a motor that applies rotational force to the valve body, and a steering control unit. The power steering system detects a steering torque applied to a steering member, and controls a hydraulic fluid pressure that is supplied to or drained from a steering assist power cylinder with the use of a hydraulic control valve that operates on the basis of the detected steering torque. An actuator for the hydraulic control valve is driven on the basis of the detected steering torque. The steering control unit computes a rotational force of the motor, which is required to generate steering assist force, on the basis of the detected steering torque obtained from a torque sensor, and supplies the motor with a current corresponding to the rotational force.
The steering assist device applies force to a steered shaft on the basis of the amount of the hydraulic fluid received via the flow rate control valve. Therefore, in order to appropriately assist the operation of a steering wheel, it is desirable to accurately control the supply amount of hydraulic fluid. However, JP 2006-306239 A does not describe that the power steering system accurately controls the supply amount of hydraulic fluid.